The role of women in Ancient Persia is a fascinating example of dichotomy. On the one hand, we have harem life and the typical ancient thought of women being possessions. On the other hand, there are records proving that women were not only allowed in the work force, they were paid TWICE the salary of a man in a similar position. Why? Probably because they were only in said position if their husband died, and so they needed the income to provide for their families. (This is my supposition—there are no facts on the “why.”) Working women were also given a year’s paid maternity leave.
Another excellent example of how esteemed women could be is Artemisia. Artemisia took control of a tyranny upon her husband’s death and went on to lead a force of ships in the fleet, become Xerxes’ most trusted adviser, and escort his sons safely back from Greece—an honor undoubtedly coveted by the men.
Given all this, and the fact that the harem had such a profound impact on the latter years of Xerxes’ reign, it’s very logical that a wife such as Kasia would have had the influence I ascribe to her. Yes, she’s totally fictional—but the authority she eventually yields would have definitely belonged to someone.